1. Field of the Invention
This invention relates generally to fault detection circuitry and particularly to detection of loss of connection between a transducer and an amplifier used to amplify the transducer output signals.
2. Description of the Prior Art
Transducers, such as thermocouples, typically produce output signals of very low level requiring amplification for use in monitoring and control systems. In such systems, it is not uncommon for the interconnections between the transducer and the amplification circuitry to include substantial wire lengths and multiple connections. Further, it is not uncommon that such connections be made by mating connector components, such as sockets and pins, which are susceptible to loss of continuity when used with low level signals. To insure the integrity of the signal produced by the amplifiers, it is desired to provide detection of loss of connection between the amplifier circuit and the transducer.
U.S. Pat. No. 4,155,080 issued 15 May 1979 to Kovacs discloses circuitry for detecting an open circuit between a thermocouple and a monitoring system. The detector uses an optical coupler having a light emitting diode driven by a power source independent of the thermocouple and a photo transistor connected with series resistance across the thermocouple output. In the arrangement disclosed in this patent, loss of connection between the transducer and the amplifier causes the current conducted by the photo transistor to charge a capacitor connected across the transducer to a level opposite in polarity to the normal condition. Since the photo transistor continuously conducts, the series resistance must be large to minimize the current through the photo transistor which appears as a load to the thermocouple output. The arrangement of this patent suffers the draw back of the relatively high cost of the optical coupler and the additional cost of providing large series resistance to limit the offset current drawn by the optical coupler. In light of the limitations of the prior art, it is desired to provide a simpler and less costly circuit for detection and loss of connection between a transducer and an amplifier.